Hearing aid performance continues to be enhanced by technological advances in audio signal processing capabilities, e.g. primarily via Digital Signal Processing, environmental adaptation, programmability, and noise cancellation techniques. An effect of such on-going efforts, suppliers of miniature acoustic transducers are often requested by hearing aid manufacturers to provide these and other performance improvements.
Today, one performance limitation for miniature electret microphones is the rather limited power supply noise immunity, i.e., Power Supply Rejection (PSR), of its internal impedance buffering circuit. A typical buffer circuit has a PSR of approximately 26 dB. To overcome the limited PSR of the electret microphone, hearing aid manufacturers can utilize either of two possible solutions: operating the microphone from a high PSR regulated power supply voltage within the hearing aid; or, filtering the battery voltage with a resistor-capacitor (RC) low-pass filter having a relatively low resistor value, e.g., approximately Kilo-ohms—to prevent a large voltage drop from the battery voltage to the circuit—along with a relatively high capacitance value, e.g., approximately tens of micro-Farads. For acceptable performance, hearing aid systems typically require at least 50 dB of PSR over the entire audio frequency band.
Unfortunately, both of these two solutions require additional components and/or circuitry in the hearing aid system and thus contribute to an undesired increase in system complexity. The increased system complexity further results in higher manufacturing costs and a greater likelihood of reliability problems in the final product.
An alternative and much more appropriate solution to the limited PSR performance would be to address the root cause of the problem and improve the PSR properties of the impedance buffering circuit. A desirable PSR solution would be one that maintains the same microphone size, battery current consumption, noise performance, and sensitivity, with little or no degradation in audio fidelity or usable dynamic range. Incorporating a voltage regulator into the microphone is not an attractive method because a larger IC is required for the new circuitry. Furthermore, additional power is required to operate the regulator. Utilizing the RC filter approach is also unacceptable because the large capacitor value that is required is physically too big to fit into the case of a miniature electret microphone.
The present invention is directed to solve theses and other problems.